It depends some on whether you're talking about the usual Wednesday downtime or the random crashes where trying anything has about a 1% chance of crashing instead of loading properly. The above posts are about the former.

What happened yesterday is that they did an overnight patch on Wednesday morning as usual, and then a few hours later, something broke so that some players were able to log in and some weren't. It took them a couple of hours to track it down, and then they took the servers offline for more than an hour to fix it. By early afternoon US Eastern time, the servers were up and working again. The posts above are from people who were unable to log in during the downtime.